Chemical-mechanical planarization (“CMP”) processes remove material from the surface of a semiconductor wafer in the production of integrated circuits. FIG. 1 schematically illustrates a CMP machine 10 having a platen 20. The platen 20 supports a planarizing medium 21 that can include a polishing pad 27 having a planarizing surface 29 on which a planarizing liquid 28 is disposed. The polishing pad 27 may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be a new generation fixed-abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid 28 may be a conventional CMP slurry with abrasive particles and chemicals that remove material from the wafer, or the planarizing liquid may be a planarizing solution without abrasive particles. In most CMP applications, conventional CMP slurries are used on conventional polishing pads, and planarizing solutions without abrasive particles are used on fixed abrasive polishing pads.
The CMP machine 10 also can include an underpad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the polishing pad 27. A drive assembly 26 rotates the platen 20 (as indicated by arrow A), or it reciprocates the platen 20 back and forth (as indicated by arrow B). Because the polishing pad 27 is attached to the underpad 25, the polishing pad 27 moves with the platen 20.
A wafer carrier 30 positioned adjacent the polishing pad 27 has a lower surface 32 to which a wafer 12 may be attached. Alternatively, the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 40 may be attached to the wafer carrier to impart axial and/or rotational motion (as indicated by arrows C and D, respectively).
To planarize the wafer 12 with the CMP machine 10, the wafer carrier 30 presses the wafer 12 face-downward against the polishing pad 27. While the face of the wafer 12 presses against the polishing pad 27, at least one of the platen 20 or the wafer carrier 30 moves relative to the other to move the wafer 12 across the planarizing surface 29. As the face of the wafer 12 moves across the planarizing surface 29, material is continuously removed from the face of the wafer 12.
One problem with CMP processing is that the throughput may drop, and the uniformity of the polished surface on the wafer may be inadequate, because waste particles from the wafer 12 accumulate on the planarizing surface 29 of the polishing pad 27. The problem is particularly acute when planarizing doped silicon oxide layers because doping softens silicon oxide and makes it slightly viscous as it is planarized. As a result, accumulations of doped silicon oxide glaze the planarizing surface 29 of the polishing pad 27 with a coating that can substantially reduce the polishing rate over the glazed regions.
To restore the planarizing characteristics of the polishing pads, the polishing pads are typically conditioned by removing the accumulations of waste matter with an abrasive conditioning disk 50. Conventional abrasive conditioning disks are generally embedded with diamond particles, and they are mounted to a separate actuator 55 on a CMP machine that can move the conditioning disk 50 rotationally, laterally, or axially, as indicated by arrows E, F, and G, respectively. Typical conditioning disks remove a thin layer of the pad material itself in addition to the waste matter to form a new, clean planarizing surface 29 on the polishing pad 27. Some conditioning processes also include disposing a liquid solution on the polishing pad 27 that dissolves some of the waste matter as the abrasive disks abrade the polishing surface.
One problem with conventional conditioning methods is that the conditioning disk 50 can lose effectiveness by wearing down or by having the interstices between abrasive particles plugged with particulate matter removed from the polishing pad 27. If the change in effectiveness is not detected, the polishing pad 27 may be insufficiently conditioned and subsequent planarizing operations may not remove a sufficient quantity of material from the wafer 12. Another problem is that the conditioning disk 50 may condition the polishing pad 27 in a nonuniform manner, for example, because the build-up of deposits on the polishing pad may be non-uniform or because the relative velocity between the polishing pad and the conditioning disk changes as the conditioning disk moves radially across the planarizing surface 29.
One approach to addressing the above problems is to measure a friction force at an interface with the polishing pad. U.S. Pat. No. 5,743,784 discloses detecting the roughness of a polishing pad with a floating head apparatus positioned away from the conditioning disk. One drawback with this method is that the friction force detected by the floating head may not accurately represent the friction force between the conditioning disk and the polishing pad. Furthermore, the separate floating head adds to the overall complexity of the CMP apparatus.
Another approach is to measure a contact force between a conditioning end effector and the polishing pad, as disclosed in U.S. Pat. No. 5,456,627. As discussed above, a drawback with this approach is that the contact force may not adequately represent the friction force between the polishing pad and the conditioner.
U.S. Pat. No. 5,036,015 discloses sensing a change in friction between the wafer and the polishing pad by measuring changes in current supplied to motors that rotate the wafer and/or the polishing pad to detect the endpoint of planarization. However, this method does not address the problem of detecting the condition of the conditioning disk.